Add duma 2.5.21 and mark 2.5.15 as obsolete. While we're at it use the
versions hosted on github which requres new checksums for the 2.5.15
version because the generated tarballs are different.
It appears we don't need any of the patches we've been carrying for the
older version but we do need to pass CC_FOR_BUILD in addition to HOSTCC.
When 2.5.15 is removed we can drop HOSTCC (and DUMA_CPP, DUMA_SO).
Signed-off-by: Chris Packham <judge.packham@gmail.com>
